1. Field
The present disclosure relates to integrated plasmon and dielectric waveguides.
2. Related Art
Future integrated photonic circuits for telecommunications and optical logic applications will require a high level of complexity. Plasmon waveguides, which are constructed out of metal, can be used to guide light in volumes far beneath the diffraction limit, offering a possible avenue towards dramatically increased device densities in integrated photonic circuits. See, for example, Takahara, J., Suguru, Y., Hiroaki, T., Morimoto, A., & Kobayashi, T., Guiding of a one-dimensional optical beam with nanometer diameter, Optics Letters 22, 475-477 (1997). A plasmon waveguide is a metal waveguide which allows conversion of the optical mode into non-radiating or only weakly radiating surface plasmons. A surface plasmon is an oscillation of free electrons that propagates along the surface of the metal. See also N. W. Ashcroft, N. D. Mermin “Solid State Physics” Brooks Cole (1976), Chapter 1, pages 19 and 27.
Compact plasmon waveguides generally suffer from high loss, and chip-scale integration presents a challenge, as does efficient coupling off-chip. See, for example, Barnes, W. L., Dereux, A., Ebbesen, T. W. Surface Plasmon Subwavelength Optics. Nature 424, 824-830 (2003).
The electromagnetic response of metals in the infrared and visible spectrum is characterized by a largely imaginary index of refraction enabling the definition of waveguides with sub-diffraction scale optical propagation. See Palik, E., Handbook of Optical Constants of Solids (Academic Press, Washington, D.C., 1985). There is a basic trade-off in all plasmon waveguide geometries between mode size and propagation loss. One can have a low propagation loss at the expense of a large mode size, such as in Nikolajsen, T., Leosson, K., Salakhutdinov, I., & Bozhevolnyi, S., Polymer-based surface-plasmon-polariton stripe waveguides at telecommunication wavelengths, Applied Physics Letters 82, 668-670 (2003), where propagation losses of 6 dB/cm for 20 nm slabs of gold, but with a 12 μnm mode diameter are reported. At the other extreme, in Takahara, J., Suguru, Y., Hiroaki, T., Morimoto, A., & Kobayashi, T., Guiding of a one-dimensional optical beam with nanometer diameter, Optics Letters 22, 475-477 (1997), guiding in 20 nm diameter silver nanowires, with a mode field diameter of about 10 nm, is predicted, but with theoretical propagation losses of 3 dB/410 nm. Though this loss is acceptable for nanoscale photonic circuitry, large scale integration with such losses is not feasible.
While efficient end-fire coupling from fiber modes to large scale plasmon waveguides has been demonstrated in Nikolajsen, a realistic path to large scale integration and off-chip coupling for nanoscale plasmon geometries has not yet been demonstrated. As a result, many of the current measurements that have been made for sub-diffraction scale plasmon optics have been done with direct interrogation methods, such as on-chip fluorescence as shown in Maier, S. A., Kik, P. G., Atwater, H. A., Meltzer, S., Harel, E., Koel, B. E., & Requicha, A. A., Local detection of electromagnetic energy transport below the diffraction limit in metal nanoparticle plasmon waveguides, Nature Materials 2, 229-232 (2003).
Plasmon based-waveguides are not the only way by which light can be guided on small scales. In particular, the applicants have previously demonstrated dielectric ridge waveguides of Silicon in Silicon-On-Insulator (SOI), which have low propagation loss of 6-7 dB/cm. See Baehr-Jones, T., Hochberg, M., Walker, C., & Scherer, A., High-Q ring resonators in thin silicon-on-insulator, Applied Physics Letters 85, (2004). Though the mode size is fundamentally diffraction limited, 90% of the optical energy is contained in a 1.5 square micron region, in such waveguides as we detail below.
Due to the low loss achievable, SOI waveguides are a promising path for chip-scale device integration. Perhaps as importantly, numerous geometries for the efficient, broadband coupling from an external fiber to an SOI waveguide have been demonstrated. See, for example, Taillaert, W. Bogaerts, P. Bienstman, T. F. Krauss, P. Van Daele, S. I. Moerman, Verstuyft, K. De Mesel, & R. Baets, An Out-of-Plane Grating Coupler for Efficient Butt-Coupling Between Compact Planar Waveguides and Single-Mode Fibers, IEEE J. Quantum Electron. 38, 949 (2002), or Almeida, V., Panepucci, R., & Lipson, M., Nanotaper for compact mode conversion, Optics Letters 28, 1302-1304 (2003).